The invention relates to a method for assigning priorities to traffic between local area networks interconnected via a backbone network.
A local area network (LAN) is a local data network taking care of traffic between workstations connected to it, such as PCs, and various devices providing services to the workstations. A basic LAN configuration comprises a physical transmission path, i.e. cabling, a network server, workstations connected to the cabling by means of adapter cards, and a network software. A LAN is typically located in a single building or in several buildings located close to each other, e.g. at one office of a specific organization or company. Recently, however, there has been an increased demand to interconnect individual LANs into larger networks. For such purposes, the above-described basic components will not suffice.
Equipments required to interconnect LANs are typically classified in accordance with the OSI (Open Systems Interconnection) model of the ISO (International Standards Organization). The OSI model aims at creating a framework for standards applied in data transmission between open systems. The model comprises seven superimposed layers the tasks of which have been specified whereas their implementation has been left open. The OSI model is described more closely, e.g. in Reference [1] (the references are listed at the end of this specification).
The devices used in the interconnection of LANs, i.e. the repeater, the bridge, and the router, will be described briefly hereinbelow.
The repeater is the simplest means used in the interconnection of LANs or LAN segments. The repeater operates on the lowest OSI layer (layer 1), i.e. on the physical layer. The repeater amplifies the bit stream and forwards all traffic over it from one network segment to another. The repeater is thus used to increase the physical length of the network, and it can be used only when the networks to be interconnected are fully identical (or differ only in the transmission medium). Segments interconnected by the repeater form a single logical network.
The bridge operates on the next OSI layer (layer 2), i.e. on the data link layer. Even though the data link layer is mainly independent of the physical transmission medium used, some of its functions are dependent on the physical transmission medium. For this reason, the data link layer includes a so-called MAC (Media Access Control) sublayer in some network architectures. The MAC sublayer provides access to the transmission path, i.e. it takes care of functions most probably associated with the characteristics of the physical transmission path. Bridges typically operate on the MAC sublayer. The function of the bridge is to monitor frames transferred over the LAN and to transfer them from one network to another on the basis of the physical address of the data packets. Only frames having a destination address indicating transmission to the side of the other network are able to cross the bridge. The bridge thus acts as an insulator which reduces the load in other network portions. The bridge does not analyze more closely what the frames transport, and it ignores the higher-level protocol transferred in the frame. In other words, the bridge is protocol-independent, and so it can be used to interconnect networks utilizing protocols of different types (TCP/IP, XNS, OSI, NetBios, etc.).
The bridge contains a so-called routing table, which is updated by the bridge on the basis of the addresses of the transmitting parties of frames received by the bridge. The routing table indicates to the bridge behind which interface a specific station is currently located. In other words, the bridge is able to "learn" station locations so that new stations can be added to the network without having to reconfigure the bridge.
The router operates on the third OSI layer, i.e. on the network layer. Routers know the higher protocols used in the LAN traffic and route messages by means of the addressing mechanisms of these protocols. The router forwards the frame (to another router or to a destination station) on the basis of the data obtained from the network address routing table. The router calculates an optimal route for each frame. The maintenance of routing data and the route selection are based on a routing protocol utilized by the router (such as RIP, Routing Information Protocol). The filtering and management properties of routers are superior to those of bridges, and they offer better possibilities for the construction and use of complicated LAN configurations.
The bridge and the router are described, e.g. in Reference [2], which is referred to for a more detailed description.
A packet switched network solution (i.e. a network over which packets of varying length are transmitted) requires matching of rates as the subscriber interfaces usually operate at different rates due to their different data transmission capacity requirements. There also often occur different rates over data links between network nodes. Rate/matching is usually performed by buffering which should not cause an excessive absolute delay in order that the applications utilizing the link would be satisfied with the data transmission service.
At present, traffic between LANs can be considered to consist of traffic of two types: terminal user traffic and datafile transfer traffic. Terminal traffic has a high interactivity requirement (a delay of 2 to 300 ms over the link in both directions) as it provides services directly to LAN users (who very often are less patient). On the contrary, the transfer of datafiles mainly consists of interequipment transfer traffic in which the required delays are not particularly critical (the delays may be several seconds).
FIG. 1 shows a public network service in which local area networks 11 of different offices A-E of one specific company are interconnected via a public network 12 acting as a backbone network. The public network 12 is any network known per se which is able to forward LAN data packets of varying length, such as a FR (Frame Relay) network. The different offices typically have different interface rates due to their different data transmission requirements, resulting from size differences between the offices. The LAN of each office is adapted to the FR service via a bridge 13 located at the office and a transmission line. The transmission lines are indicated with the references 14a-14e, respectively. Such interconnection is described in more detail in Pyhalammi et al, U.S. patent application Ser. No. 08/416,682, filed Apr. 5, 1995, which is referred to for a more detailed description.
The link between the offices A and B will be described by way of example below. The office B transmits first a packet burst to the office A. As the office LAN operates at a high rate (e.g. 10 Mbit/s), rate/matching is required for the subscriber line 14b having a capacity of 64 kbit/s in this example (FIG. 2). The matching is performed by the bridge 13 located within the premises of the office B, by buffering the packets and forwarding them at an access rate to the network 12. Busses within the network 12 have a higher capacity than the subscriber lines, and so there is no need for buffering there. In this case, the rate of the subscriber line of the office A (2 Mbit/s) is also higher than that of the office B, so that the office A is able to receive the packets at the rate at which they were transmitted from the office B.
Problems occur when the situation is reversed, i.e. when the rate of the transmitting interface is higher than that of the receiving interface (e.g. traffic from the head office to a branch office). In such a case, the entire packet burst passes from the subscriber interface A over the network 12 to the edge of the network, and further to the interface of the office B, where the packets have to be buffered to wait for access to the subscriber line. This situation is illustrated in FIG. 2, where a buffering point is indicated by the reference P and a buffer by the reference numeral 21. In practice, the problem is not the buffering, as such, but the delay caused by the buffering as the delay disturbs the interactive work. For instance, if the access rate is 64 kbit/s, the delay of a single packet of one kilobyte will be 125 ms. If a plurality of such packets are transmitted concurrently, as usual, the delay will be intolerable for the interactive operation. This is because if a packet requiring a rapid response is received after this kind of packet burst, it has to wait in a queue for the buffer 21 to be emptied.